The present invention relates to the field of thermal recording or printing and, more specifically to a thermal recording system and method for recording grey scale images on thermally sensitive recording media.
Thermal printing potentially is a very attractive process for providing low cost "hard copies" of electronically recorded images. Unlike expensive silver halide based photographic reproduction materials, thermally sensitive paper or the like is relatively inexpensive.
To date, this potential has not been fulfilled because certain aspects of the thermal process have not been controlled adequately to produce acceptable tonal or grey scale renditions. While this process works well in the high contrast binary mode (black or white) to print sharp dot matrix alphanumeric characters or line drawing graphics, attempts to introduce grey scale, for example to reproduce a photographic image, have been disappointing.
Conventional half-tone lithographic printing processes, both color and black and white, are capable of producing high quality photograph-like tonal image reproductions with resolutions in the range of 150 to 300 lines per inch. The printed image is formed by a matrix array of pixels, each having an assigned tonal density within a grey scale that extends between white and black or between very light and very dark colors. Each printed pixel comprises one pixel area in a grid or matrix of uniformly sized areas on the print receiving sheet (e.g. white paper) and either no dot or a dot of varying size printed therein. If no dot is printed, incident light is reflected from the entire pixel area to represent the lowest density (brightest) or lightest pixel in the grey scale. When a very small dot is printed in the pixel area, the eye perceives the area as a very light grey and it may have a density of about 5% in the grey scale. A larger dot covering approximately half the pixel area is perceived as a neutral grey or a 50% density pixel. When the dot size is increased still further to fill or almost completely fill the pixel area, the pixel is perceived as a black or a 95% to 100% density pixel.
The quality of the image production increases with increasing grey scale range. Range, in turn, depends on the ability to accurately vary dot size in discrete narrow steps between a minimum and a maximum size. In commercial quality lithography, it is not uncommon for the grey scale to have 128 or more steps which is indicative of just how well dot size is controlled.
By analogy, high quality grey scale images may be reproduced by thermal printing if the process is capable of forming dots or marks which can be made to vary in size or area on an accurately controlled basis to provide an acceptable range of pixel densities.
In contrast to the high resolution reproduction capabilities of lithographic printing or photography, thermal recording tends to be a low resolution process because it is difficult to establish control over the many variable process parameters.
For example, in a typical thermal dot matrix printer, a sheet of paper having a thermally sensitive layer thereon is placed between a thermal print head, confronting the layer, and a rubber roller or platen which presses against the backside of the sheet to establish pressure contact between the head and layer. The print head typically comprises an array of electrically resistive elements which generate heat when current is passed therethrough. Each element is configured to be switched on and off individually and specified elements are turned on simultaneously to form a corresponding character or part of a character. The head may have a single array of a few elements and be indexed horizontally across the paper to print a line, or it may consist of a linear array disposed in a horizontal row across the width of the paper to print a full line at a time.
The thermally sensitive coating generally includes a heat sensitive dye that is very light or transparent at room temperature and changes color or becomes dark when exposed to thermal energy which exceeds a threshold temperature. Typically, these dyes react at temperatures in the range of 140.degree. to 300.degree. F. Once the dye reaction threshold temperature has been exceeded, a dot or mark begins to appear and it progressively grows in size in response to the continued application of thermal energy. Typically, the dot grows to fill the entire pixel area contacted by an individual element, but blooming, or growth beyond the element boundaries, may occur if the heat is applied too long.
We will assume for the moment that the thermal paper is consistent in its response to heat exposure over its entire area, which generally is not the case.
The goal in dot matrix printing is to make each dot of uniform size in density. If each resistive element in the head array is not of exactly the same resistivity, there will be variations in heat output among the elements which are subjected to the same supply voltage, and this works against uniform dot size.
If, during the course of the printing process the supply voltage varies, the thermal output of the heat elements will change and result in uneven dot production.
Residual heat and thermal inertia are other variable parameters that present major obstacles to accurately controlling the thermal output of each individual heat element in the array. For example, assume that there are seven elements in a vertical array. Suppose that to print the first part of the character it is required that all seven elements be energized, and to print the next adjacent part of the character, only one of the seven is to be energized. When all of the elements are energized simultaneously, there is maximum heat build up in the array. While the array is designed to cool down rapidly once the current is turned off, there is the possibility that there will be some residual heat therein when the next electrical impulse is applied to actuate the single element. That means that the single element will overshoot the design temperature and not produce a dot of the desired size and density.
In addition to variations in supply voltage, resistivity of the individual elements, and residual heat problems, other variable parameters associated with thermal printing which may work against producing uniform dots include possible variations in pressure contact between the head and the paper, due to local variations of resiliency in the rubber roller or platen, and variations in the scanning rate at which the head is moved across the paper. If the scanning rate slows down, heat application time increases thereby increasing the cumulative thermal energy applied to a pixel area which results in the production of a larger dot then when the scanning rate is faster and less total heat is applied.
In the preceding discussion of the variable parameters associated with the thermal printer, it was assumed that the thermal recording medium was consistent in thermal response over its entire area. Experience has shown that in actual practice, one can expect variations in heat response from sheet to sheet taken from the same box, and it is not uncommon to find variations in response on the same sheet due to uneven coating thickness or non-uniform chemical response at different locations in the coating. Also, variations in the thickness of the base paper over the course of one sheet, and from sheet to sheet, tend to cause changes in the pressure contact the print head makes with the thermally sensitive layer which again tends to work against producing uniform dot size.
Thermal recording systems known in the prior art acknowledge the existence of the above noted variable parameters and disclose a variety of control systems designed to improve the uniformity of dot size or line width.
For example U.S. Pat. Nos. 4,407,003 and 4,442,342 disclose control systems wherein a sensor detects the amount of voltage applied to the print head and, via a feedback loop, adjusts the time the print head applies heat to a given location to insure that a uniform amount of thermal energy is applied to form each dot.
U.S. Pat. No. 3,577,137 discloses a thermal printer control system wherein the temperature of the print head is sensed by a diode located in an adjacently mounted integrated circuit immediately prior to the next print cycle, and the power applied to drive the head during the print cycle is adjusted in accordance with this reading. This system compensates for residual heat and applies consistent amounts of thermal energy to each location to form dots of a uniform size.
U.S. Pat. No. 4,412,229 discloses an XY plotter which utilizes a laser printhead. This patent acknowledges that when the head moves slowly the recorded line is thicker than when the head moves rapidly because of the difference in the amount of thermal energy applied at the different head velocities. Using a control system, head velocity is anticipated and the power input to the laser is varied accordingly (i.e. higher voltage for high velocity) to produce a uniform line.
U.S. Pat. No. 4,064,205 discloses a laser printing system for making thermal plastic lithographic printing plates. A plastic plate is initially covered with a transfer sheet of infrared absorbing material and is scanned to replicate an original document. After scanning, the transfer sheet is stripped away leaving the absorbing material on selected areas of the plate to be depressed by melting when exposed to an infrared beam. In one embodiment, Col. 4, Line 60, a sensor reads the sheet just ahead of the scanning beam looking for the infrared absorbing material. If the sensor output signal indicates the absence of infrared absorbing material, the laser beam is maintained at a low energy output state to inhibit melting. When the sensor does detect the infrared material, the laser beam output is automatically switched from low to high to selectively melt or deform the covered area.
U.S. Pat. No. 4,355,318 is directed to a laser data recording system wherein an opaque plastic medium is rendered transparent in spots where it is selectively heated with the laser beam. A photocell is placed on the underside of the medium to sense whether or not a desired transparent spot has been created by comparing the photocell reading with the input signal to the recording laser. An error signal indicates that data has not been properly recorded either because of a data error or an imperfection in the recording material. In another embodiment, the photocell looks at a recording medium just ahead of the laser beam recording track to sense and identify pin hole defects in the medium.
All of these above noted prior art thermal printing systems utilize some type of sensor to detect a variable parameter and a feedback loop to make adjustments or compensations in the process based on the conditions detected by the sensor. However these disclosures generally are directed to achieving dot or line uniformity and do not address the proposition of providing variable density pixels by adjusting the size of the dots printed therein to provide a halftone-like grey scale reproductive capability.
Therefore, it is an object of the present invention to provide a thermal recording system and method for making tonal or grey scale images on a thermally sensitive recording medium.
Another object is to provide a thermal recording system and method capable of producing variable density pixels on a thermally sensitive medium.
Yet another object is to provide such a system and method that is configured to receive electronic image signals and to reproduce a visual image, in accordance with the signals, on a thermally sensitive medium.
Another object is to provide such a system and method that is capable of producing dots of various size, in an accurately controlled manner, on a thermally sensitive medium.
Other objects of the invention will, in part, be obvious and will, in part, appearinafter.